1. Field of the Invention
The present invention generally relates to methods and apparatus for the continuous drilling of a wellbore through an earth formation. More particularly, the present invention pertains to the continuous circulation of fluid through two tubulars that are being connected or disconnected during a wellbore drilling operation. In addition, embodiments of the present invention relate to continuously rotating and axially advancing two drill pipes into a wellbore while circulating drilling fluid through the two drill pipes and forming a connection between the two drill pipes.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. The wellbore extends from the earth's surface to a selected depth in order to intersect a hydrocarbon-bearing formation. In many drilling operations, the drill string comprises a plurality of “joints” of drill pipe that are threadedly connected at the platform of the drilling rig. As the wellbore is formed at lower depths or more extended intervals, additional joints of pipe are added at the platform. These joints are then rotated and urged downwardly in order to form the wellbore.
During the drilling process, drilling fluid is typically circulated through the drill string and back up the annular region formed by the drill string and the surrounding formation. As the drilling fluid is circulated, it exits ports, or “jets,” provided in the drill bit. This circulation of fluid serves to lubricate and cool the bit, and also facilitates the removal of cuttings and debris from the wellbore that is being formed.
One common method for providing rotation to the drill string involves the use of a kelly bar. The kelly bar is attached to the top joint of the drill string, and is driven rotationally by means of a rotary table at the derrick floor level. At the same time, the kelly bar is able to move vertically through a drive bushing within the rotary table at the rig floor. An alternative method for imparting rotation to the drill string uses a top drive that is hung from the derrick and is capable of gripping the drill string and rotating it. In such an arrangement, a kelly bar is not required.
As the drill bit penetrates into the earth and the wellbore is lengthened, more sections of hollow tubular drill pipe are added to the top of the drill string. This involves stopping the drilling, i.e., rotational and axial translation of the drill pipe, while the successive tubulars are added. The process is reversed when the drill string is removed. Drill string removal is necessary during such operations as replacing the drilling bit or cementing a section of casing. Interruption of drilling may mean that the circulation of the mud stops and has to be re-started when drilling resumes. Since the mud is a long fluid column, the resumption of circulation throughout the wellbore can be time consuming. Such activity may also have deleterious effects on the walls of the wellbore being drilled, leading to formation damage and causing problems in maintaining an open wellbore.
Intermittent cessation of fluid circulation may require additional weighting of the mud. In this respect, a particular mud weight must be chosen to provide a static head relating to the ambient pressure at the top of a drill string when it is open while tubulars are being added or removed. The additional weighting of the mud to compensate for cessation of fluid circulation adds expense to the operation.
One purpose of fluid circulation while drilling relates to the suspension of cuttings. To convey drilled cuttings away from a drill bit and up the wellbore, the cuttings are maintained in suspension in the drilling fluid. When the flow of fluid ceases, such as when adding or removing a section of drill pipe, the cuttings tend to fall down through the fluid. To inhibit cuttings from falling out, the drilling mud is further weighted, and viscosity is reduced. The use of thicker drilling fluids requires more pumping power at the surface. Further, the act of “breaking” the pumps to restart fluid circulation following a cessation of circulation may result in over pressuring of a downhole formation. This can trigger formation damage or even a loss of fluids downhole, endangering the lives of the drilling crew due to loss of hydrostatic pressure. Of course, the additional weighting of drilling mud adds expense to the drilling operation.
Systems and methods for continuously circulating fluid through two tubulars that are being connected or disconnected are disclosed in U.S. Pat. No. 6,412,554. The '554 patent is assigned to Weatherford/Lamb, Inc. The '554 patent is incorporated herein by reference, in its entirety. The systems and methods of the '554 patent allow for continuous fluid circulation during the drilling operation; however, rotation of the drill string must still be stopped and re-started in order to connect and disconnect the tubulars. Therefore, valuable time loss occurs when drilling stops in order to connect the next successive section of drill pipe. Additionally, starting rotation of the drill string can over torque portions of the drill string, causing failure from the additional stress.
U.S. Pat. No. 6,315,051 discloses methods and apparatus for both continuously rotating a tubular string and continuously circulating fluid through the tubulars as sections of pipe are added or removed. However, inability to continue to advance the tubular string down the borehole during the connection process temporarily stops drilling into the formation. The wellbore forming process is thus stopped temporarily in order to make up or break out the successive pipe connections.
Therefore, there is a need for efficient methods and apparatus for connecting and disconnecting tubular sections while at the same time rotating and axially translating a tubular string there below, and while continuously circulating fluid through the tubular string.